Is QLC SSD worth buying?

With the development of solid-state disk, NAND flash has updated four times which begin with SLC, then MLC and TLC in recent years, finally QLC.

Firstly, let’s learn some about SLC, MLC, TLC and QLC. 
SLC is the best, the longest life circle and the fastest speed, which has been widely known by the public, but the shortcomings of SLC are also obvious. The price of SLC is very expensive and the capacity is too small, it has basically withdrawn from the consumer market. So now normally we cannot find SLC particles of the solid-state disk in ordinary market. Such as bulk Micro SD 128MB for GPS, 512MB and 1GB, these small capacity are mostly using SLC solution.

MLC is equivalent to double cells of SLC, the capacity of MLC is also twice as SLC. In another words, each storage cell of MLC is divided into two halves based on SLC. The benefits of MLC are obvious, and the common capacity can generally reach 256GB. Although Micro SD card wholesaler can’t use MLC solution to produce 256GB, the working principle is same. With MLC NAND flash, the capacity of Micro SD cards in bulk is double size as SLC.

Although the capacity of MLC has become much larger, but its speed gets slower and the life circle have been shorter, therefore it is still far to meet the increasing demand for daily use.

So, TLC solution just emerges in time, the capacity of it increases again, it can achieve 512GB or above, and the current mass production of SSD with TLC is up to 2TB, but the speed and life circle is not as good as the previous two predecessors. 

The speed and life circle issue lead to the low demands for some consumers. However, according to the current use habits and the basic quality of TLC particles, in fact we do not have to worry about the life circle of TLC particles of the solid-state disk.

What is QLC？
The concept of QLC may be unfamiliar with most consumers. Let’s make brief introduction here. QLC is Quad-Level Cell, each cell can storage 4 bits data. Compared to TLC NAND flash memory, the storage density of QLC NAND flash increased by 33%. A QLC flash memory cell has as many as 16 voltage states, which puts forward high requirements on the quality of flash memory and error correction ability of controller. It is easy to understand that QLC can extend higher capacity, the capacity of QLC will be equivalent to the four times of SLC, it basically can achieve the TB level to start, but the speed and life circle will be comparatively slower and shorter.

It is easy to understand that QLC can extend higher capacity, the capacity of QLC will be equivalent to the four times of SLC, it basically can achieve the TB level to start, but the speed and life circle will be comparatively slower and shorter.

The arrival of QLC SSD is like TLC SSD, which is resisted by consumer either at first. TLC SSD came out when there were SLC SSD and MLC SSD everywhere after all, which had many flaws including fewer erase number, lower speed than MLC and uncertainty of whether the stored data could be guaranteed or not. Due to various controversy, the resistance of TLC is reasonable which explains the worry of QLC.

Given the huge problems that TLC SSD caused when it was first released, I wish that QLC SSD will not cause any problems when it was released. Intel 660p SSD had released for a year. Although there is not big problem, the performance of it is desperate.

If looking at the 660p SSD specification given by Intel, you may think it is just a slower SSD. After all, it is not uncommon that SSDs with continuous read and write speed of 1800MB/s. However, smart Intel only tells us the write speed in the SLC Cache rather than the original write speed of QLC after using.

Intel SSD 660p Specification

According to the sheet above, the performance of it is no bad and the price of per GB is quite low, isn't it very attractive? However, it uses SLC Cache for write acceleration just like the TLC SSD. Otherwise, the original write speed is much slower. But the original write speed of QLC SSD is a little surprisingly slow. So 660p use a large SLC Cache to avoid the speed crash when the Cache runs out.

The SLC Cache with large capacity is dynamic. At the start, there are nothing in the SSD so Cache space is large, but Cache space will decrease with the decrease of available space. When the SSD usable space is dropped to the lowest, SLC Cache of 512 GB SSD is 6 GB, that of 1TB is 12GB, that of 2 TB is 24 GB. Maybe it seems okay. After all, now some SSDs has 7 ~ 8G SLC Cache. But what is really collapsed is the speed after it runs out of SLC Cache.

The Performance test results of Intel 660p 1TB

The original write speed of Intel 660p 512GB QLC is only about 50MB/s, 1TB is 100MB/s and 2TB is 150MB/s. The 512GB QLC is even worse than HDDs and the latter two are not much better. As for those who think the cache is too big to run normally, I also thought so until I tested the ROG 3 notebook.

To test performance of this laptop, we're going to copy the game into it. It is all known that the size of the game is now small with capacity ranging from 10 GB to 300 GB. It usually takes 10 minutes for M.2 SSD to copy this file. This laptop has a 1 TB 660p SSD, the write speed is quick at the beginning while the speed is 100 MB/s after SLC Cache run out. It took me about two hours, which refreshed my understanding of QLC SSD.

So, is QLC SSD worth buying?
512GB QLC SSD and 1TB QLC SSD is not recommended to buy, because the former can be replaces with TLC SSD at same price and the latter can add a little money to buy PM981 1TB, so they are not recommended. Although 2 TB 660p SSD with poor performance, it is so cheap that it is feasible to use it as a storage disk to replace HDD especially the game volume is increasingly large now. The performance of 660p SSD isn't nearly as good as that of TLC SSDs, but it's still pretty good compared to HDDs. Using QLC SSD as the system disk is not an advisable choose.
The QLC SSDs on the market generally use the 64-layer stacking process that TLC flash use, without apparent price advantage.
Although QLC flash memory is theoretically not as durable as TLC in writing, QLC flash SSDs have larger capacity. Under a relatively fixed write load, the consumption speed of flash write life slows as capacity increases. Overall, QLC SSD is more suitable for home read-based, light write-based conditions.

As the increasing QLC capacity, its durability will be proved as time goes by. QLC with capacity over 1TB and TLC SSD for small capacity is recommended. Now that giants like Samsung and Intel start launch their QLC it means its technologies are mature enough with the age of QLC flash memory era. It will be the same with TLC SSD being launched by more brands. HDD, which is the most affected, in notebook will be firstly weeded out by standard QLC. Thus, the purchase of QLC SSD will be guaranteed if only brand reputation is good.

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